The present invention relates to a preamble structure for the synchronization of a receiver of a OFDM transmission. The invention furthermore relates to an OFDM transmitter as well as to a method for the synchronization of a receiver of an OFDM transmission system.
With reference to FIG. 2 now an autocorrelation technique on the receiving side of an OFDM system will be explained. The received signal is delayed by a delaying unit 2 by the correlation delay Dac. The conjugate complex samples of the delayed version of the signals are generated 3 and multiplied 4 with the received samples. The products are set into the moving average unit 6 with a window size Wac and are then postprocessed for a threshold detection and/or maximum search (units 5, 7, 8) to find the correct timing. The complex correlation result at the peak possession generated by the unit 9 can be used to estimate the frequency offset.
A synchronization preamble structure as shown in FIG. 1 is known. This known synchronization preamble structure can be subdivided in a A-FIELD, B-FIELD and a C-FIELD. The A-FIELD and the B-FIELD are subdivided in further parts. Each of the A-FIELD and the B-FIELD and the C-FIELD is designed to have an optimized special synchronization function at the receiving side. The A-FIELD for example serves for a coarse frame detection and an automatic gain control (AGC). The B-FIELD serves as a coarse frequency offset and timing synchronization. The C-FIELD serves for a channel estimation and fine synchronization.
Details about the concrete structure and generation of the B-FIELD can be found in the European patent application 99 103 379.6 in the name of Sony International (Europe) GmbH, which is to be regarded as representing prior art according to article 54(3) EPC. Regarding the details of the B-FIELD and generally the generation of the time domain synchronization preamble signal as shown in FIG. 1 reference is made to said prior non-prepublished application.
The symbols of the C-FIELD, which is generally of minor interest for the present invention, are defined in frequency domain as
C64xe2x88x9226 . . . 26={1,1,xe2x88x921,xe2x88x921,1,1,xe2x88x921,1,xe2x88x921,1,1,1,1,1,1,xe2x88x921,xe2x88x921,1,xe2x88x921,1,1,xe2x88x921,1,1,1,1,0, 1,xe2x88x921,xe2x88x921,1,1,xe2x88x921,1,xe2x88x921,1,xe2x88x921,xe2x88x921,xe2x88x921,xe2x88x921,xe2x88x921,1,1,xe2x88x921,xe2x88x921,1,xe2x88x921,1,xe2x88x921,1,1,1,1}
The symbols B16 of the B-FIELD are short OFDM symbols, of which the subcarriers +xe2x88x924, +xe2x88x928, +xe2x88x9212, +xe2x88x9216, +xe2x88x9220, +xe2x88x9224 are modulated.
The content in the frequency domain is defined as:
B16xe2x88x9226 . . . 26=sqrt(2)*{0,0,1+j,0,0,0,xe2x88x921+j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1xe2x88x92j,0,0,0,0,0,0,0,1xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,xe2x88x921+j,0,0,0,1+j,0,0}
The last repetition of the B-FIELD in the time domain, called IB16, is a sign inverted copy of the preceding B16.
The symbols A16 are short OFDM symbols, of which the subcarriers +xe2x88x922, +xe2x88x926, +xe2x88x9210, +xe2x88x9214, +xe2x88x9218, +xe2x88x9222, are modulated. The content in the frequency domain is defined as:
Axe2x88x9226 . . . 26=0,0,0,+1xe2x88x92j,0,0,0,+1+j,0,0,0,xe2x88x921+j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,+1xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,+1xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,+1xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,xe2x88x921+j,0,0,0,+1+j,0,0,0,0}
The sign reversal of every second A16 symbol in the time domain is automatically achieved by the specified subcarrier loading. The last repetition of the A-FIELD in time domain, called IA16, is a copy of the preceding RA16.
An optimized matching between A and B-FIELD of the BCCH preamble is achieved as shown in FIG. 3 and thus the timing accuracy improvement, which is basically achieved through the specified time domain structure, is kept. Two clear single AC amplitude peaks can be identified in the BCCH preamble. Additionally a tow plateau in front of the second AC peak can be seen, which is advantageous for receiver synchronization processing (e.g. used as threshold to invoke correlation peak search algorithm).
In the last time a new B-FIELD was proposed. In the following this new B-FIELD will be explained.
The symbols B16 according to this new B-field are short OFDM symbols, of which the subcarriers +xe2x88x924, +xe2x88x928, +xe2x88x9212, +xe2x88x9216, +xe2x88x9220, +xe2x88x9224 are modulated.
B16xe2x88x9226 . . . 26=sqrt(2)*{0,0,1+j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1+j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1+j,0,0,0,0,0,0,0xe2x88x921xe2x88x92j,0,0,0,xe2x88x921xe2x88x92j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0}
This new B-field results in improved performance when using cross-correlation based receivers due to lower cross-correlation sidelobes at the border from the B-FIELD to the C-FIELD.
The short OFDM symbols, consisting of 12 modulated subcarriers are phase modulated by the elements of the symbol alphabet S={square root over (2)}(xc2x11xc2x1j). The C-FIELD symbols are not considered here.
The generalized mapping for field B is:
Sxe2x88x9226,26=sqrt(2)*{0,0,S1,0,0,0,S2,0,0,0,S3,0,0,0,S4,0,0,0,S5,0,0,0,S6,0,0,0,0,0,0,0,S7,0,0,0,S8,0,0,0,S9,0,0,0,S10,0,0,0,S11,0,0,0,S12,0,0}
where xe2x80x98sqrt(2)xe2x80x99 is used to normalize the power. Applying a 64-point IFFT to the vector S, where the remaining 15 values are set to zero xe2x80x98fourxe2x80x99 short training symbols can be generated. The IFFT output is cyclically extended to result in the dedicated number of short symbols.
The generalized mapping for field A is:
Sxe2x88x9226,26=sqrt(2)*{0,0,0,0,S1,0,0,0,S2,0,0,0,S3,0,0,0,S4,0,0,0,S5,0,0,0,S6,0,0,0,S7,0,0,0,S8,0,0,0,S9,0,0,0,S10,0,0,0,S11,0,0,0,S12,0,0,0,0}
Where xe2x80x98sqrt(2)xe2x80x99 is used to normalize the power. Applying a 64-point IFFT to the vector S, where the remaining 15 values are set to zero xe2x80x98fourxe2x80x99 short training symbols can be generated. The IFFT output is cyclically extended to result in the dedicated number of short symbols.
The currently specified sequence for field A is:
S1 . . . 12=(+1xe2x88x92j), (+1+j), (xe2x88x921+j), (xe2x88x921xe2x88x92j), (+1xe2x88x92j), (xe2x88x921xe2x88x92j), (+1xe2x88x92j), (xe2x88x921xe2x88x92j), (+1xe2x88x92j), (xe2x88x921xe2x88x92j), (xe2x88x921+j), (+1+j)
Using the new B-FIELD no optimization has been made in the A-FIELD in order to improve auto-correlation based receiver synchronization.
FIG. 4 shows the ideal AC result (amplitude and phase) using a BCCH preamble structure with unmodified A-FIELD, C-FIELD and the new (modified) B-FIELD based on the B16 sequence proposed. The AC result is used to identify a frame start, adjust the AGC and to do timing and frequency synchronization. Especially the B-FIELD can be used for the later synchronization tasks. It is very important to achieve time synchronization as accurate as possible. With the configuration described two auto-correlation peaks (A-FIELD, modified B-FIELD) are visible, however, the slopes on both sides of the B-FIELD peak are very different (steep gradient on the right, shallow gradient on the left), this effect decreases the synchronization accuracy significantly. Additionally a high plateau can be seen before the auto-correlation peak in field B (samples 105 . . . 125). This effect decreases the detection performance.
The above set forth latest proposed B-FIELD and A-FIELD combination has a disadvantage that when using the new B-FIELD no optimization has to be made in the A-FIELD in order to prove the auto-correlation properties of the corresponding receiver synchronization. The sequence to be used in the A-FIELD should additionally have a minimum Peak-to-Average-Power-Ratio (PAPR) and a small dynamic range (DR).
In view of the above disadvantages of the prior art, it is the object of the present invention to propose A-FIELD sequences which are optimized regarding the time domain signal properties.
It is a further object of the present invention to propose A-FIELD sequences which are optimized regarding the resulting auto-correlation based receiver synchronization characteristics when using the latest proposes B-FIELD sequence.
According to a first aspect of the present invention therefore a preamble structure for the synchronization of a receiver of a OFDM transmission system is proposed. The preamble comprises at least one first part. The at least one first part is designed f.e. for a coarse frame detection and/or a AGC control. The at least one first part contains inverse fast fourier transformed frequency domain sequences of complex symbols. The time domain signal of synchronization preamble is generated by mapping frequency domain sequences of 12 complex symbols to a 64-point IFFT according to the following scheme:
Sxe2x88x9226,26=sqrt(2)*{0,0,0,0,S1,0,0,0,S2,0,0,0,S3,0,0,0,S4,0,0,0,S5,0,0,0,S6,0,0,0,S7,0,0,0,S8,0,0,0,S9,0,0,0,S10,0,0,0,S11,0,0,0,S12,0,0,0,0},
wherein the remaining valued are set to zero.
The frequency domain sequence SA of the at least one first part (with the appropriate A-FIELD mapping as set forth above) is one of
S1 . . . S12=+A,+A,+A,+A,+A,xe2x88x92A,xe2x88x92A,+A,+A,xe2x88x92A,+A,xe2x88x92A
S1 . . . S12=+A,+A,+A,+A,xe2x88x92A,xe2x88x92A,+A,+A,xe2x88x92A,+A,xe2x88x92A,+A
S1 . . . S12=+A,+B,xe2x88x92A,xe2x88x92B,xe2x88x92A,xe2x88x92B,xe2x88x92A,xe2x88x92B,xe2x88x92A,+B,+A,xe2x88x92B
S1 . . . S12=+A,+B,xe2x88x92A,xe2x88x92B,+A,xe2x88x92B,+A,xe2x88x92B,+A,xe2x88x92B,xe2x88x92A,+B
S1 . . . S12=+A,xe2x88x92B,xe2x88x92A,+B,xe2x88x92A,+B,xe2x88x92A,+B,xe2x88x92A,xe2x88x92B,+A,+B, or
S1 . . . S12=+A,xe2x88x92B,xe2x88x92A,+B,+A,+B,+A,+B,+A,+B,xe2x88x92A,xe2x88x92B
or an order reversed modification thereof.
The above sequences are also advantageous in case a preamble structure having only one part is used as the time domain signal properties are already improved by said sequences alone.
A second part (B-field) can be provided, wherein the frequency domain sequence of the at least one second part corresponds to the above captioned latest proposed B-field sequence, i.e.:
SB=(1+j), (xe2x88x921xe2x88x92j), (1+j), (xe2x88x921xe2x88x92j), (xe2x88x921xe2x88x92j), (1+j), (xe2x88x921xe2x88x92j), (xe2x88x921xe2x88x92j), (1+j), (1+j), (1+j), (1+j).
Particularly the A-field sequences
S1 . . . S12=+A,xe2x88x92B,xe2x88x92A,+B,xe2x88x92A,+B,xe2x88x92A,+B,xe2x88x92A,xe2x88x92B,+A,+B, or
S1 . . . S12=+A,xe2x88x92B,xe2x88x92A,+B,+A,+B,+A,+B,+A,+B,xe2x88x92A,xe2x88x92B
or an order reversed modification thereof, in combination with said B-filed sequence result in improved autocorrelation characteristics.
The at least one second part can follow the at least one first part in the time domain.
According to a further aspect of the present invention an OFDM transmitter designed for transmitting a synchronization preamble as set forth in the BCCH channel of an OFDM system is provided.
According to a still further aspect of the present invention a method for the synchronization of a receiver of a OFDM transmission system is provided.
Further advantages, features and objects of the present invention will become evident for the man skilled in the art by means of the following description of embodiments of the present invention taken into conjunction with the figures of the enclosed drawings.